Optical transceiver and method of controlling optical powers of optical channels

ABSTRACT

Systems and methods for controlling optical powers of optical channels in an optical communications network comprising a plurality of nodes is described herein. The method comprises obtaining a reference optical power. The method also includes determining an optical power of an optical channel generated by an optical transmitter of a node. The method further includes applying an attenuation to the optical channel to reduce the optical power of the optical channel to the reference optical power. In some implementations, the method is performed by a network controller operating in the optical communications network.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/329,301, filed on Feb. 28, 2019, which is a 35 U.S.C. § 371 nationalstage application of PCT International Application No.PCT/SE2016/050853, filed on Sep. 13, 2016, the disclosure and content ofwhich is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The invention relates to an optical transceiver and to a radio basestation, RBS, node comprising the optical transceiver. The inventionfurther relates to a radio base station, RBS, an optical communicationsnetwork and a method of controlling optical powers of optical channelsin an optical communications network.

BACKGROUND

The cellular architecture that is widely used by conventional mobilecommunication networks may include four basic elements for end-to-end,E2E, data transport and managed communication: the user equipment, UE,the radio access network, RAN, the core network, CN, and the operation,administration and maintenance, OAM, system. Currently deployed globalsystem for mobile communication, GSM, wideband code-division multipleaccess, WCDMA, and universal mobile telecommunications system, UMTS, theso-called the 2nd generation, 2G, and the 3rd generation, 3G, mobilecommunication networks, conventionally use RANs comprising twogeographically separated sites: radio base stations, RBS; and RBScontrollers, for example a base station controller BSC and a radionetwork controller, RNC. After launching the 4th generation, 4G, longterm evolution, LTE, mobile network, two separated sites belonging the2G and 3G legacy RAN have been merged into a single site, the eNode B,eNB, comprising 2 sub-sites: a sub-site with baseband units, BBU, and asub-site with the radio units, RU.

Various types of topologies for the field deployment of fronthauloptical network over legacy 2G/3G/4G networks have been recommended inthe common public radio interface, CPRI, specification. One populartopology is the main-remote topology which has a star-network layout inwhich a BBU is remotely connected to a number of remote radio units,RRU, with a typical link distance of between a few hundred meters and 2km. The link paths between the BBU and the RRUs are known as CPRI links.Conventionally, point-to-point, P2P, of single mode fibre, SMF, basedduplex or simplex connections have been used by CPRI links. Thefrequency bands used by RU/RRU are usually below 3 GHz, which may enableimplementation of flexible bandwidths from a few hundred KHz up to 20MHz. With the use of arrayed antennas, for example a 4×4 antenna array,a data transmission rate of up to of 300 Mbit/s can be achieved betweenRRUs and UEs. To be able to support multiple RRUs with a flexiblecombination of different bandwidths for field deployment of RBSs, asystem bandwidth of 100 MHz-200 MHz is commonly used to design legacyBBUs.

With the rapid growth of mobile communications in recent years, mobilecommunication systems are now required to support much larger systemcapacities with higher data rates over large coverage areas in ahigh-mobility environment. To satisfy such demands, a 5th generationmobile network, 5G, has been recently proposed. One of the basicrequirements for the 5G network being outlined by the standardizationbodies is that the 5G network shall deliver various types of services toUEs with ultrahigh peak data rates, e.g. tens of Gbit/s peak data ratesfor both uplink and downlink transmissions.

In order to achieve the desired ultrahigh peak data rates, a high-degreearrayed antenna system, for example 8×8 or 16×16 antenna arrays, and a5G-Radio with ultra-wide system bandwidth, e.g. over 1 GHz, may be used.This is because the peak transmission data rate increases withincreasing the system bandwidth as well as the number of arrayedantennas. To be able to design a compact 5G-Radio, it is desired todirectly integrate arrayed antenna system into the 5G-Radio. Since thesize of the antenna element decreases with increasing operatingfrequency, the 5G-Radio may be designed to be operated at the highfrequency bands, for example 28 GHz.

Alternatively, the CPRI line bit rate of 10 Gbit/s with system bandwidthof 100-200 MHz conventionally designed for 2G/3G/4G-enabled legacy BBUsmay be adapted for 5G-enabled BBUs, particularly during early stagefield development of 5G-Radios that need to be integrated into thelegacy 2G/3G/4G radio networks. Therefore, in order to satisfy theultra-wide bandwidth of 5G-Radio, a number of BBUs designed for thelegacy LTE, 4G, network may be used to provide bandwidth aggregation toimplement the 5G-Radio. Taking 800 MHz bandwidth 5G-Radio as an example,one may make use of eight 100 MHz LTE-BBUs or four 200 MHz LTE-BBUs toimplement bandwidth aggregation in order to provide 800 MHz bandwidthfor 5G-radio.

One of challenges for the field deployment of the 5G-Radio network isthe mismatch in optical distribution network, ODN, topology between thelegacy 2G/3G/4G and the new 5G networks. In contrast to a conventionalBBU-centralized star topology used by CPRI transport within 2G/3G/4Gsites, the 5G-Radio now becomes the centralized point in the startopology where a single 5G-Radio has to be connected to a number of BBUsfor CPRI transport. One of major problems to deploy a 5G-Radio networkover and/or on top of legacy 2G/3G/4G networks is the significantincrease of number of fibres, which could be up to a factor of 10 ormore. For example, consider an 8 transmitter/receiver duplex-SMFinterfaced transceiver is designed for 5G-Radio, and for the sake ofbandwidth aggregation, a 5G-Radio centralized star topology is used tocross-connect a 5G-Radio with a cluster of four BBUs. With the mostsimple site configuration of 3-sectors and a single branch, 48 SMFs willbe needed in order to support data steam transport over CPRI links forthree 5G-Radios, which is a factor of 8 increase in the number of fibrescompared to a similar 3-sector site with 6 SMFs in 2G/3G/4G cases. Sucha drastic increase in the of number fibres for a single site is notacceptable by mobile operators due to the extreme high cost of the fibreroll-out and/or the cost of leased fibres in the existing 2G/3G/4G radionetworks.

One of well-known methods to reduce the number of fibres is to make theuse of dense wavelength division multiplexing, DWDM, technologies. UsingDWDM, it is possible to reduce a large number of SMFs down to a singleSMF. DWDM technologies also enable the deployment of cascaded-chainand/or ring network topologies for fronthaul optical networks, withtransport link protection. Unfortunately, the commercial availableoff-the-shelf key components used by DWDM technologies, for example,transponders, arrayed waveguide gratings, AWG, wavelength selectiveswitches, WSS, erbium-doped fibre amplifiers, EDFA, etc., are veryexpensive. This is because these components are usually designed tosatisfy highly demanding requirements in terms of providing high linkbudget, high thermal stability and high flexibility for channel-planswith the possibility of specific band-bypass/band-filtering etc. and aredesigned for long-haul transport networks.

SUMMARY

It is an object to provide an improved optical transceiver. It is afurther object to provide an improved radio base station, RBS, node. Itis a further object to provide an improved radio base station, RBS. Itis a further object to provide an improved optical communicationsnetwork. It is a further object to provide an improved method ofcontrolling optical powers of optical channels in an opticalcommunications network.

A first aspect of the invention provides an optical transceivercomprising an optical waveguide, a first add-drop port, a secondadd-drop port, an optical transmitter and an optical receiver. The firstadd-drop port is at a first end of the optical waveguide and the secondadd-drop port is at a second end of the optical waveguide. The opticaltransmitter is operable to generate an optical channel at a respectivewavelength. The optical transmitter is coupled to a reconfigurableoptical channel-add apparatus. The reconfigurable optical channel-addapparatus comprises a first optical add path including a first opticalattenuator, a second optical add path including a second opticalattenuator, and an add micro-ring resonator. The first opticalattenuator and the second optical attenuator are reconfigurable toselectively block an optical channel from the optical transmitter in oneof the first optical add path and the second optical add path. The addmicro-ring resonator is reconfigurable selectively to add an opticalchannel from the first optical add path to the optical waveguide totravel towards the first add-drop port or to add an optical channel fromthe second optical add path to the optical waveguide to travel towardsthe second add-drop port. The optical receiver is coupled to areconfigurable optical channel-drop apparatus. The reconfigurableoptical channel-drop apparatus comprises a drop micro-ring resonator, afirst drop path, and a second drop path. The drop micro-ring resonatoris reconfigurable selectively to drop an optical channel travelling fromthe first add-drop port from the optical waveguide to the first droppath or to drop an optical channel travelling from the second add-dropport from the optical waveguide to the second drop path.

This structure may enable reconfiguration of the optical transceiverbetween a working mode and a protection mode, which may enable aprotection mechanism to be implemented in an RBS and in an opticalcommunications network, as described below. The optical transceivertherefore has a built-in switching functionality and can thus be usedfor the deployment of link protection mechanisms. A transceiver havingan embedded ROADM is therefore provided which may enable theconstruction of DWDM-enabled transport solutions for fronthaul opticalnetworks with low energy consumption, low-cost and small footprint.

In an embodiment, the first optical attenuator and the second opticalattenuator are reconfigurable between a first state in which an opticalchannel from the optical transmitter is blocked in the first optical addpath and a second state in which an optical channel from the opticaltransmitter is blocked in the second optical add path. The addmicro-ring resonator is reconfigurable between a first state in whichthe add micro-ring resonator is configured to add an optical channelfrom the second optical add path to the optical waveguide to traveltowards the second add-drop port and a second state in which the addmicro-ring resonator is configured to add an optical channel from thefirst optical add path to the optical waveguide to travel towards thefirst add-drop port. The drop micro-ring resonator is reconfigurablebetween a first state in which the drop micro-ring resonator isconfigured to drop an optical channel travelling from the secondadd-drop port from the optical waveguide to the second drop path and asecond state in which the drop micro-ring resonator is configured todrop an optical channel travelling from the first add-drop port from theoptical waveguide to the first drop path. The optical transceiveradditionally comprises a controller configured to receive a secondcontrol signal and to cause the first optical attenuator and the secondoptical attenuator, the add micro-ring resonator and the drop micro-ringresonator to switch between the first state and the second state independence on the second control signal. This structure may enablereconfiguration of the optical transceiver between a working mode and aprotection mode, which may enable a protection mechanism to beimplemented in an optical communications network, as described below.

The controller could be implemented as one or more processors, hardware,processing hardware or circuitry.

References to processors, hardware, processing hardware or circuitry canencompass any kind of logic or analog circuitry, integrated to anydegree, and not limited to general purpose processors, digital signalprocessors, ASICs, FPGAs, discrete components or logic and so on.References to a processor are intended to encompass implementationsusing multiple processors which may be integrated together, orco-located in the same node or distributed at different locations forexample.

In an embodiment, the optical transceiver comprises a plurality ofoptical transmitters, a plurality of reconfigurable optical channel-addapparatus, and a plurality of optical receivers. Each opticaltransmitter is operable to generate a respective optical channel at arespective one of a plurality of wavelengths. Each optical transmitteris coupled to a respective reconfigurable optical channel-add apparatus.Each optical receivers is coupled to a respective one of the pluralityof reconfigurable optical channel-drop apparatus.

A multi-wavelength optical transceiver is therefore provided. Bycoupling each transmitter to a micro-ring resonator based reconfigurableoptical channel-add apparatus and each receiver to a micro-ringresonator reconfigurable optical channel-drop apparatus, themulti-wavelength optical transceiver requires only a single opticaltransmitter for each optical channel, a single optical receiver for eachoptical channel, and a single optical waveguide bus. This structure mayenable reconfiguration of the optical transceiver between a working modeand a protection mode, which may enable a protection mechanism to beimplemented in an RBS and in an optical communications network, asdescribed below. This structure may also reduce the number oftransmitters, receivers and connectors required by the transceiver andsimplify the structure of the transceiver, allowing it to have a smallersize. A multi-wavelength transceiver having an embedded ROADM istherefore provided which may enable the construction of DWDM-enabledtransport solutions for fronthaul optical networks with low energyconsumption, low-cost and small footprint, having a reduction by afactor of 5 or more compared to the transport solutions withconventional DWDM devices.

In an embodiment, the first optical attenuator and the second opticalattenuator of each reconfigurable optical channel-add apparatus areadditionally reconfigurable to apply an optical attenuation to anoptical channel in the other of the first add path and the second addpath. This may enable regulation of the optical power for each of theoptical channels so that when the optical transmitter is implemented inan RBS node within a FrON, crosstalk between adjacent channels in theFrON may be diminished. Unbalanced incoming optical powers in DWDM-basedFrON, which may be generated for different DWDM channels during theirpropagation over the DWDM link paths, may therefore be reduced. This mayalso reduce degradation of signal quality signal extraction andanalysis, which a significant difference in optical powers can cause dueto poor resolution caused by the superposition of signals for adjacentchannels.

In an embodiment, the optical transceiver additionally comprises opticalattenuator control apparatus configured to generate a first controlsignal comprising an indication of respective optical attenuations to beapplied in the optical channel-add apparatuses. Each optical attenuationdepends on an optical power of the optical channel generated by therespective optical transmitter and depends on a reference optical powerfor the plurality of optical channels. This may enable static and/ordynamic regulation of the optical power for all the optical channelsagainst a common reference, so that crosstalk between adjacent channelsin a fronthaul optical network may be diminished. Dependence of theoptical attenuation on a reference optical power may also enable theoptical power of each of the optical channels to be adjusted such thatit is the same as the other optical channels transmitted over thefronthaul optical network.

In an embodiment, each add micro-ring resonator is arranged such that anoptical channel received from the first add path passes in a clockwisedirection around at least a portion of the add micro-ring resonator, andan optical channel received from the second add path passes in ananticlockwise direction around at least a portion of the add micro-ringresonator. The optical transceiver therefore requires only a singleoptical transmitter for each optical channel, a single optical receiverfor each optical channel, and a single optical waveguide bus. Thisstructure may enable reconfiguration of the optical transceiver betweena working mode and a protection mode, which may enable a protectionmechanism to be implemented in an optical communications network, asdescribed below.

In an embodiment, each reconfigurable optical channel-add apparatusadditionally comprises an optical coupling apparatus having an inputconfigured to receive an optical channel from the optical transmitter, afirst output coupled to the first add path and a second output coupledto the second add path. An optical channel can therefore be providedfrom the respective optical transmitter to the same input, whether theoptical is to be added from the first add-drop port or from the secondadd-drop port.

In an embodiment, the optical coupling apparatus comprises an opticalpower splitter configured to split the received optical channel into afirst optical channel which is output from the first output and a secondoptical channel which is output from the second output.

In an embodiment, the first add path comprises a second opticalwaveguide and the second add path comprises a third, different opticalwaveguide.

In an embodiment, in each reconfigurable optical channel-drop apparatusthe first drop path includes third optical attenuator and the secondoptical drop path includes a fourth optical attenuator. The thirdoptical attenuator and the fourth optical attenuator are reconfigurableto selectively attenuate an optical channel received from the dropmicro-ring resonator.

In an embodiment, each reconfigurable optical channel-drop apparatusadditionally comprises an optical coupling apparatus having a firstinput configured to receive an optical channel from the first drop pathand a second input configured to receive an optical channel from thesecond drop path and an output coupled to the optical receiver.

In an embodiment, the first drop path comprises a fourth opticalwaveguide and the second drop path comprises a fifth, different opticalwaveguide.

In an embodiment, the optical transceiver is implemented as a photonicintegrated circuit, PIC, and may be implement as an integrated, singlesilicon photonic device. Providing a photonically enabled multi-λ DWDMTRX with an embedded ROADM may enable deployment of DWDM-enabledtransport solutions for fronthaul optical networks with low energyconsumption, low-cost and small footprint.

In an embodiment, the optical transceiver is packaged in one of apluggable form factor and a board-mounted form factor.

Corresponding embodiments are also applicable to the radio base station,RBS, node, to the radio base station, RBS, and to the opticalcommunications network described below.

A further aspect of the invention provides a radio base station, RBS,node comprising an optical transceiver comprising an optical waveguide,a first add-drop port, a second add-drop port, an optical transmitterand an optical receiver. The first add-drop port is at a first end ofthe optical waveguide and the second add-drop port is at a second end ofthe optical waveguide. The optical transmitter is operable to generatean optical channel at a respective wavelength. The optical transmitteris coupled to a reconfigurable optical channel-add apparatus. Thereconfigurable optical channel-add apparatus comprises a first opticaladd path including a first optical attenuator, a second optical add pathincluding a second optical attenuator, and an add micro-ring resonator.The first optical attenuator and the second optical attenuator arereconfigurable to selectively block an optical channel from the opticaltransmitter in one of the first optical add path and the second opticaladd path. The add micro-ring resonator is reconfigurable selectively toadd an optical channel from the first optical add path to the opticalwaveguide to travel towards the first add-drop port or to add an opticalchannel from the second optical add path to the optical waveguide totravel towards the second add-drop port. The optical receiver is coupledto a reconfigurable optical channel-drop apparatus. The reconfigurableoptical channel-drop apparatus comprises a drop micro-ring resonator, afirst drop path, and a second drop path. The drop micro-ring resonatoris reconfigurable selectively to drop an optical channel travelling fromthe first add-drop port from the optical waveguide to the first droppath or to drop an optical channel travelling from the second add-dropport from the optical waveguide to the second drop path.

The RBS node may enable the field deployment of 5G-Radios and basebandunits, BBUs, over legacy radio access networks.

In an embodiment, the RBS node additionally comprises first control unitconfigured to generate the second control signal in response toreceiving a protection control signal comprising an indication thatreconfiguration is required between the working mode and the protectionmode.

A further aspect of the invention provides a radio base stationcomprising a remote radio unit, RRU, comprising a first opticaltransceiver, a baseband unit, BBU, comprising a second opticaltransceiver, a first optical fibre link and a second optical fibre link.The first optical transceiver and the second optical transceiver eachcomprise an optical waveguide, a first add-drop port, a second add-dropport, an optical transmitter and an optical receiver. The first add-dropport is at a first end of the optical waveguide and the second add-dropport is at a second end of the optical waveguide. The opticaltransmitter is operable to generate an optical channel at a respectivewavelength. The optical transmitter is coupled to a reconfigurableoptical channel-add apparatus. The reconfigurable optical channel-addapparatus comprises a first optical add path including a first opticalattenuator, a second optical add path including a second opticalattenuator, and an add micro-ring resonator. The first opticalattenuator and the second optical attenuator are reconfigurable toselectively block an optical channel from the optical transmitter in oneof the first optical add path and the second optical add path. The addmicro-ring resonator is reconfigurable selectively to add an opticalchannel from the first optical add path to the optical waveguide totravel towards the first add-drop port or to add an optical channel fromthe second optical add path to the optical waveguide to travel towardsthe second add-drop port. The optical receiver is coupled to areconfigurable optical channel-drop apparatus. The reconfigurableoptical channel-drop apparatus comprises a drop micro-ring resonator, afirst drop path, and a second drop path. The drop micro-ring resonatoris reconfigurable selectively to drop an optical channel travelling fromthe first add-drop port from the optical waveguide to the first droppath or to drop an optical channel travelling from the second add-dropport from the optical waveguide to the second drop path. The firstoptical fibre link and the second optical fibre link are each coupledbetween the RRU and the BBU. The optical transmitter of the firstoptical transceiver is operable to generate an optical channel at afirst wavelength and the optical transmitter of the second opticaltransceiver is operable to generate an optical channel at a secondwavelength, different to the first wavelength.

The RRU and BBU are therefore point-2-point connected by two redundantoptical fibre links; one optical fibre link may form a working mode linkand the other optical fibre link may form a protection mode link. TheRBS may therefore implement a link protection mechanism by reconfiguringthe optical transceivers. The optical transceiver having built-inswitching functionality that can support two bi-directional wavelengthsover one of two redundant optical fibre links, either the working linkor the protecting link, can thus be used for the deployment of linkprotection mechanisms for point-2-point connected two node systems.

A further aspect of the invention provides an optical communicationsnetwork comprising a bi-directional wavelength division multiplexing,WDM, ring interconnecting a plurality of first RBS nodes. At least oneof the first RBS nodes is a baseband unit, BBU, and at least one otherof the first RBS nodes is a remote radio unit, RRU. Each first RBS nodecomprises an optical transceiver comprising an optical waveguide, afirst add-drop port, a second add-drop port, a plurality of opticaltransmitters and a plurality of optical receivers. The first add-dropport is at a first end of the optical waveguide and the second add-dropport is at a second end of the optical waveguide. Each opticaltransmitter is operable to generate a respective optical channel at arespective one of a plurality of wavelengths. Each optical transmitteris coupled to a respective reconfigurable optical channel-add apparatus.The reconfigurable optical channel-add apparatus comprises a firstoptical add path including a first optical attenuator, a second opticaladd path including a second optical attenuator, and an add micro-ringresonator. The first optical attenuator and the second opticalattenuator are reconfigurable to selectively block an optical channelfrom the optical transmitter in one of the first optical add path andthe second optical add path. The add micro-ring resonator isreconfigurable selectively to add an optical channel from the firstoptical add path to the optical waveguide to travel towards the firstadd-drop port or to add an optical channel from the second optical addpath to the optical waveguide to travel towards the second add-dropport. Each optical receiver is coupled to a respective reconfigurableoptical channel-drop apparatus. The reconfigurable optical channel-dropapparatus comprises a drop micro-ring resonator, a first drop path, anda second drop path. The drop micro-ring resonator is reconfigurableselectively to drop an optical channel travelling from the firstadd-drop port from the optical waveguide to the first drop path or todrop an optical channel travelling from the second add-drop port fromthe optical waveguide to the second drop path.

The communications network has an architecture which may enable thefield integration of 5G-Radios into existing, legacy 2G/3G/4G-Radionetworks. This may enable a smooth migration of legacy mobile networksto the next-generation of mobile networks, i.e. 5G mobile network andbeyond. The communications network may support bandwidth de-aggregationconventionally configured between a single legacy BBU and a number oflegacy RRUs but also support bandwidth aggregation between a 5GBBU-cluster and a single 5G-Radio. The communications network maysignificantly simplify the fibre-infrastructure for the field deploymentand/integration of 5G-Radios into legacy 2G/3G/4G-Radio networks.

In an embodiment, the optical communications network additionallycomprises a plurality of second RBS nodes connected to the WDM ring viaa plurality of reconfigurable optical add drop multiplexers, ROADMs.Each second RBS node comprises a single-wavelength optical transceiver.At least one of the second RBS nodes is a baseband unit, BBU, and atleast one other of the second RBS nodes is a remote radio unit, RRU.

In an embodiment, the WDM ring is a dense WDM, DWDM, ring.

In an embodiment, the first RBS nodes are nodes of a 5G radio networkand the second RBS nodes are nodes of one of a 2G, 3G and 4G radionetwork. The network may provide a DWDM-ring based fronthaul opticalnetwork architecture/topology to support the field deployment of5G-Radios into legacy 2G/3G/4G-Radio networks.

In an embodiment, the optical communications network additionallycomprises a first control unit configured to obtain a reference opticalpower for the communications network and wherein the optical attenuatorcontrol apparatus in the first RBS nodes are configured to receive athird control signal comprising an indication of the reference opticalpower.

In an embodiment, the first control unit is configured to: obtain atransmission loss for each optical channel of each of the RBS nodes;identify the optical channel having the maximum transmission loss; andset the reference optical power equal to an optical power of the opticalchannel having the maximum transmission loss.

In an embodiment, the optical communications network additionallycomprises a second control unit configured to provide a protectioncontrol signal in response to an indication that a fault has occurred inthe optical communications network.

In an embodiment, the optical communications network additionallycomprises an operations, administration and management, OAM, systemconfigured to perform in-band OAM signalling with the first RBS nodesand configured to perform out-band OAM signalling with the second RBSnodes. The use of dedicated “in-band” and “out-band” OAM services tosupport the implementation static and dynamic processes for powerregulation to diminish the unbalanced power induced crosstalk, which maysignificantly enhance the quality of signals during their propagationover the WDM ring network.

In an embodiment, the optical communications network is a fronthauloptical network of a radio access network, RAN. The communicationsnetwork may enable a symmetrical layout of a fronthaul optical networkwith extended star-topologies for legacy RBS site & 5G RBS site over theWDM-Ring network.

Corresponding embodiments are also applicable to the method describedbelow.

A further aspect of the invention provides a method of controllingoptical powers of optical channels in an optical communications networkcomprising a plurality of nodes. The method comprises: obtaining areference optical power; determining an optical power of an opticalchannel generated by an optical transmitter of a node; and applying anattenuation to the optical channel at the optical transmitter to reducethe optical power of the optical channel to the reference optical power.

This may enable static and/or dynamic regulation of the optical powerfor each of the optical channels generated at a node against a commonreference so that crosstalk between adjacent channels in the network maybe diminished. Dependence of the optical attenuation on a referenceoptical power may also enable the optical power of each of the opticalchannels generated at the node to be adjusted such that it is the sameas other optical channels transmitted over the optical network.Unbalanced incoming optical powers in, for example, a DWDM-based opticalnetwork, which may be generated for different DWDM channels during theirpropagation over the DWDM link paths, may therefore be reduced. This mayalso reduce degradation of signal quality signal extraction andanalysis, which a significant difference in optical powers can cause dueto poor resolution caused by the superposition of signals for adjacentchannels.

In an embodiment, the reference optical power is obtained by: obtaininga transmission loss for each optical channel of each of the nodes;identifying the optical channel having the maximum transmission loss;and setting the reference optical power equal to an optical power of theoptical channel having the maximum transmission loss. This may enablestatic and/or dynamic regulation of the optical power for all theoptical channels in an optical network against a common reference, sothat crosstalk between adjacent channels may be diminished.

In an embodiment, the transmission loss is for each optical channel ofeach of the nodes a static transmission loss. The method may be appliedfor compensating static loss sources, for example adding and removingnodes in link paths of the WDM ring.

A further aspect of the invention provides a computer program,comprising instructions which, when executed on at least one processor,cause the at least one processor to perform any of the above steps ofthe method of controlling optical powers of optical channels in anoptical communications network comprising a plurality of nodes.

A further aspect of the invention provides a data carrier havingcomputer readable instructions embodied therein. The said computerreadable instructions are for providing access to resources available ona processor. The computer readable instructions comprise instructions tocause the processor to perform any of the above steps of the method ofcontrolling optical powers of optical channels in an opticalcommunications network comprising a plurality of nodes.

In an embodiment, the carrier is one of an electronic signal, opticalsignal, radio signal, or computer readable storage medium.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical transceiver according to an embodiment ofthe invention;

FIG. 2 illustrates an optical transceiver according to an embodiment ofthe invention;

FIG. 3 illustrates an optical transceiver according to an embodiment ofthe invention;

FIG. 4 illustrates an optical transceiver according to an embodiment ofthe invention;

FIG. 5 illustrates an optical transceiver according to an embodiment ofthe invention;

FIG. 6 illustrates a radio base station, RBS, node according to anembodiment of the invention;

FIG. 7 illustrates a radio base station, RBS, according to an embodimentof the invention;

FIG. 8 illustrates an optical communications network according to anembodiment of the invention;

FIG. 9 illustrates an optical communications network according to anembodiment of the invention;

FIG. 10 illustrates an optical communications network according to anembodiment of the invention;

FIG. 11 illustrates steps of a method according to an embodiment of theinvention of controlling optical powers of optical channels in anoptical communications network comprising a plurality of nodes;

FIG. 12 illustrates steps of a method according to an embodiment of theinvention of controlling optical powers of optical channels in anoptical communications network comprising a plurality of nodes;

FIG. 13 illustrates an optical communications network comprising aplurality of nodes;

FIG. 14 illustrates steps of a method according to an embodiment of theinvention of controlling optical powers of optical channels in theoptical communications network of FIG. 13; and

FIG. 15 illustrates steps of a method according to an embodiment of theinvention of controlling optical powers of optical channels in theoptical communications network of FIG. 13.

DETAILED DESCRIPTION

The same reference numbers will used for corresponding features indifferent embodiments.

Referring to FIG. 1, an embodiment of the invention provides an opticaltransceiver 10 comprising an optical waveguide 12, a first add-drop port14 at a first end of the optical waveguide and a second add-drop port 16at a second end of the optical waveguide. The optical transceiveradditionally comprises a plurality of optical transmitters 18 and aplurality of optical receivers 32, such that the optical transceiver 10is a multi-wavelength optical transceiver.

Each optical transmitter, Tx, is operable to generate a respectiveoptical channel at a respective one of a plurality of wavelengths. Eachoptical transmitter is coupled to a respective reconfigurable opticalchannel-add apparatus 20. Each optical receiver, Rx, is coupled to arespective reconfigurable optical channel-drop apparatus 34.

Each reconfigurable optical channel-add apparatus 20 comprises a firstoptical add path 22 including a first optical attenuator 24, a secondoptical add path 26 including a second optical attenuator 28, and an addmicro-ring resonator 30. The first optical attenuator and the secondoptical attenuator are reconfigurable to selectively block an opticalchannel from the optical transmitter in either the first optical addpath or the second optical add path. The add micro-ring resonator isreconfigurable either to add an optical channel from the first opticaladd path to the optical waveguide to travel towards the first add-dropport or to add an optical channel from the second optical add path tothe optical waveguide to travel towards the second add-drop port.

Each reconfigurable optical channel-drop apparatus 34 comprises a dropmicro-ring resonator 36, a first drop path 38, and a second drop path40. The drop micro-ring resonator is reconfigurable either to drop anoptical channel travelling from the first add-drop port from the opticalwaveguide to the first drop path or to drop an optical channeltravelling from the second add-drop port from the optical waveguide tothe second drop path.

In another embodiment, illustrated in FIG. 2, the first opticalattenuator 24 and the second optical attenuator 28 of eachreconfigurable optical channel-add apparatus 60 are additionallyreconfigurable to apply an optical attenuation to an optical channel inthe other of the first add path and the second add path.

In operation, if the first optical attenuator 24 is configured to blockthe optical channel propagating in the first optical add path, thesecond optical attenuator 28 will be configured to apply an opticalattenuation to the optical channel propagating in the second optical addpath. The first and second optical attenuators can then be reconfiguredinto the opposite arrangement, so the second optical attenuator 28 isconfigured to block the optical channel propagating in the secondoptical add path, the first optical attenuator 24 is configured to applyan optical attenuation to the optical channel propagating in the firstoptical add path. By configuring all of the reconfigurable opticalchannel-add apparatus in the same way, at any one time the opticaltransceiver can be reconfigured between a working mode and a protectionmode.

In a further embodiment, each optical attenuator 24, 28 is a variableoptical attenuator, VOA. The optical transceiver 50 additionallycomprises optical attenuator control apparatus, TX-VOA Ctrl, 52configured to generate a first control signal comprising an indicationof respective optical attenuations to be applied by the VOAs in theoptical channel-add apparatuses 60. Each optical attenuation depends onan optical power of the optical channel generated by the respectiveoptical transmitter and depends on a reference optical power for theplurality of optical channels. The optical power of each optical channelmay be obtained by tapping part of the optical channel after the VOA ineach optical add-path and measuring the tapped signal at photodetectionapparatus comprising a photodiode 56 and an analogue-to-digitalconverter, ADC, 54.

In a further embodiment, each optical attenuator 24, 28, in eachreconfigurable optical channel-add apparatus, is reconfigurable betweena first state in which an optical channel from the optical transmitteris blocked in the first optical add path and a second state in which anoptical channel from the optical transmitter is blocked in the secondoptical add path.

Each add micro-ring resonator 30 is reconfigurable between a first statein which the add micro-ring resonator is configured to add an opticalchannel from the second optical add path to the optical waveguide totravel towards the second add-drop port and a second state in which theadd micro-ring resonator is configured to add an optical channel fromthe first optical add path to the optical waveguide to travel towardsthe first add-drop port

Each drop micro-ring resonator 36 is reconfigurable between a firststate in which the drop micro-ring resonator is configured to drop anoptical channel travelling from the second add-drop port from theoptical waveguide to the second drop path and a second state in whichthe drop micro-ring resonator is configured to drop an optical channeltravelling from the first add-drop port from the optical waveguide tothe first drop path.

In this embodiment, the optical transceiver 50 additionally comprises acontroller (not illustrated) configured to receive a second controlsignal and, in dependence on the second control signal, to cause thefirst optical attenuator and the second optical attenuator of eachreconfigurable optical channel-add apparatus, the add micro-ringresonators and the drop micro-ring resonators to switch between thefirst state and the second state.

A further embodiment of the invention provides an optical transceiver100 as shown in FIG. 3, which again is a multi-wavelength opticaltransceiver and is substantially the same as the previous embodiment.The optical transceiver 100 of this embodiment is implemented as anintegrated, single photonic integrated circuit, PIC.

FIG. 3 is a simplified bock diagram of the photonically enabledintegrated circuit, PIC, of the multi-wavelength, multi-λ, opticaltransceiver 100. The PIC may use different types of generic platformsfor the monolithic integration of various types of optoelectronic andmicroelectronic devices. The platform may be either a silicon photonic,SiP, based platform or an indium phosphide, InP, based platform, whichmay be fully compiled to the well-known manufacturing processesspecified for complementary metal-oxide-semiconductor, CMOS. Varioustypes of optoelectronic and microelectronic devices, may be eitherdirectly fabricated on semiconductor-on-insulator, SOI, wafer utilizingthe CMOS processes or fabricated as external chipsets to be integratedonto the PIC utilizing the well-known flip-chip manufacturing processes.A hybrid SiP/InP multi-chip integrating platform may be used tofabricate the PIC if the optoelectronic and microelectronic devices arein compatible with the fabricating processes used by either a singleSiP-based platform or a single InP-based platform.

The PIC multi-wavelength optical transceiver, TRX, 100 may be packagedin one of various well-known small form factor pluggable modules, suchas CFP/CFP2/CFP4/CFP8/QSFP28 or may be designed as a board-mountedmodule using proprietary types of form factors for PIC packaging.

The multi-λ DWDM TRX is equipped with multi-λ DWDM transmitters TXi(λk)and multi-λ receivers RXj(λl), where the indexes; i=1, 2, . . . , M andj=1, 2, . . . , N; are optical channel marks, which may be staticallyassigned to the transmitters and the receivers belonging to themulti-λDWDM TRX, and the indexes; k=1, 2, . . . , m, 1=1, 2, . . . , nand k≠l; are wavelength marks. For the transmitter channels, TXi(λk),the wavelengths λk may be statically assigned to the multi-λDWDM TRXaccording to a pre-defined channel-plan. For the receiver channels,RXj(λl), any of the channel wavelengths may be dynamically assigned toany of desired channels during channel-drop operations carried by theembedded ROADM. The wavelengths assigned to the transmitters must bedifferent from those assigned to the receivers, and no wavelength re-useis possible.

The TXi(λk) 80 comprise: high power continuous wave, CW, lasers(λk) 82;fibre-to/from-waveguide couplers, FWC, that may be the grating type ofcoupler or the mode-field matching type of coupler for vertical or edgeassembly of the fibre onto PIC; optical waveguides for transverseelectric ground mode, OWG-TE; optical power splitters, OPS; opticalsignal modulators, OSM, which in this example are Mach-Zehnderinterferometers, MZI, comprising a pair of phase modulators, PM; opticalpower combiners OPC; optical power taps, OPT.

The reconfigurable optical channel-add apparatus comprises a2-directional reconfigurable optical channel-add multiplexer ROCAM,2D-ROCAM, 84. The 2D-ROCAM 84 comprises: an optical power splitter, OPS;2 OWG-TEs forming the first (‘West’) add path 86 and the second (‘East’)add path 90, including 2 built-in variable optical attenuators, TX-VOA,88, 92 for transmitter power regulation; and an add micro-ringresonator, MRR(λk), 100.

The receivers, RXj(λl), 110 of the multi-λDWDM TRX 100 support droppingof channels or bypassing channels that are not to be dropped to theRXj(λl). Generally speaking, the RXj(λl) may have the same amount ofchannel number as that of TXi(λk).

Each RXj(λl) comprises a receiver photodiode, RX-PD, and is coupled to areconfigurable optical channel-drop apparatus, which in this embodimentis a 2-directional reconfigurable optical channel-drop multiplexerROCDM, 2D-ROCDM. The 2D-ROCDM comprises: an OPC; and two forming thefirst (West′) drop path 114 and the second (East′) drop path 118,including two variable optical attenuators, RX-VOA, 116, 120 forreceiver power regulation; a drop micro-ring resonator 130, MRR(λl). TheRX-PDs may be wideband devices that can cover an entire wavelength band,for example the C-band, so that the RX-PDs can support the selectivedrop of any desired wavelengths to any desired receiver channels RXj(λl)via the 2D-ROCDM 112.

The optical waveguide 12 is an OWG-TE connected at each end to two FWCto support channel-add operations on both the first (‘West’) add-dropmultiplexer, ADM, port 72 and the second (‘East’) ADM port 74respectively, and to support channel-drop operations on both the WESTADM port and the EAST ADM port respectively.

Each ADM port 72, 74 is coupled to the respective FWCs via two SMFs.Each ADM port comprises a 3-way optical circulator which splits incomingtraffic and/or combines outgoing traffic. Due the nature of circulatordesign, it can also reject any backward propagating signals to avoidrecycling of dropped channels back out of the optical transceiver.

The transceiver 100 additionally comprises: TX electrical signal drivercircuitry, TX-Driver; ADCs; TX-VOA digital control circuitry, TX-VOACtrl; RX electrical signal driver circuitry, RX-Driver; RX electricalsignal amplifier circuitry, RX-Amplifier; RX-VOA digital controlcircuitry, RX-VOA Ctrl; heaters for controlling the add and dropmicro-ring resonators, indicated by dashed line boxes; digital-to-analogconverters, DAC; and digital control circuitry for the heaters,Heater-Ctrl. To enable communication with local and/or remote hostsystems, a built-in microprocessor, memory device and digitalinput-output, I/O, interface for the control and transport of digitalsignals are also provided in the PIC.

The CW lasers may be either the arrayed InP type of high power chiplasers that may be directly embedded into CMOS photonic die and/or anoff-chip fibre-pigtailed distributed feedback, DFB, type of high powerlaser that may be designed as the flipped-attached devices integratedonto the PIC. The pigtailed fibre used by the DFB lasers may bepolarization maintaining fibre, PMF. Because of strong built-inbirefringence of PMF, the corresponding polarization status of laserbeam in terms of the transverse electric, TE, mode propagated along theslow-axis and the transverse magnetic, TM, mode propagated along thefast-axis will be maintained. By carrying the pre-alignment between thepolarization-axis of laser beam and the slow-axis of pigtailed PMF aswell as between the slow-axis of pigtailed PMF to the axis of FWC duringfabricating and/or packaging processes, the optical power of laser beampropagated inside the OGW-TE may be maximized.

The centre wavelengths of the CW lasers may be selected according to aproprietary channel-plan and/or a standardized channel-plan specified bystandardization bodies, for example ITU-T G.694.1 06/2002, which belongsto one of the SMF transmission windows, e.g. C-band or L-band, etc.

The MZI enable modulation of incoming digital signals over the opticalchannel signals provided by the CW lasers 82, via the PM devices in theMZI arms. The optical signal provided by each individual CW laser isfirst coupled into the FWC through the PMF, and then guided into theOWG-TE. Since both the FWC and OWG-TE may be designed to only supportthe TE ground mode, the residual of high-order TE modes and/or TM modesshall be effectively filtered. Thus, the single TE ground mode isobtained. After passing through FWC, the optical signal propagates tothe optical inputs of the two PMs of the MZI. Via the digital I/Ocircuit, the inputs of differential digital signals TXpi and TXni, wherei=1, 2, . . . , m; p & n stand for the pin-out for positive and negativepolarities; the digital signals from the host system shall be loaded onto the PIC and delivered to the TX-Driver for signal processing. TheTX-Driver directs the processed signals into the electrical inputs ofPMs for optical signal modulation. The modulated optical signals fromthe outputs of PMs are optically combined by the OPC. At the OPT a smallportion of the optical signals are tapped off and guided to the Ctrl-PD56. The remainder of the optical channel signal is transported to the2D-ROCAM 84.

The components/devices belonging to 2D-ROCAM may be directly fabricatedand embedded on the photonic die of the PIC. The incoming optical beamfrom the MZI is split into two link paths, i.e. the West Path and theEast Path, and enters the two built-in photonic TX-VOAs respectively.According to the instructions given by TX-VOA Ctrl, the TX-VOA can applyany desired values of attenuation on the optical signals passing throughit. Thus, the outgoing optical power can be turned down to any desiredlevel. This enables the optical power for all outgoing channels TXi(λk)to be dynamically balanced, by actively monitoring and comparing theoptical powers extracted by all Ctrl-PDs.

During the normal operation of the multi-λ TRX 100, outgoing opticalchannels may only be guided either on the WEST ADM Port or the EAST ADMPort. For example, if channels are to be added from the WEST ADM port,the maximum attenuation will be set by the right TX-VOA 92 on the “East”add path 90 to completely terminate the beam onto the EAST ADM port 74.Similarly, channels are to be added from the EAST ADM Port, the maximumattenuation will be set by the left TX-VOA 88 on the “WEST” add path 86,to completely terminate the beam onto the WEST ADM port. In order toeffectively terminate unwanted channels, the TX-VOA is used as theoutgoing beam terminators for the multi-λ TRX.

After passing through the TX-VOA, the MRR(λk) selectively directs eitherthe optical channel from the west link path onto the outgoing path ofWEST ADM port or the optical channel from the east link path onto theoutgoing path of EAST ADM port. The specific channel-add operation forthe MRR(λk) is done by applying a suitable current on the heater, whichis controlled by Heater Ctrl via the high precise DAC. Furthermore, withthe suitable combination of the four FWC and two OPS, it allows thebypass of optical channels that are not to be dropped by 2D-ROCDM on theEAST ADM port or the WEST ADM port. For example, if the incoming beam ison the WEST ADM Port, the beam will be directed onto the most left FWCthrough the duplex adapter where the beam will be split and guided intothe 2D-ROCDM for the channel drop, the channels that are not droppedwill be bypassed to the outgoing port of the EAST ADM port via thefurthest right FWC. Similarly, if the incoming beam is on the EAST ADMport, besides the enabling of channel drop by the 2D-ROCDM, the channelsthat are not dropped will be bypassed and directed onto the WEST ADMport via the 2nd left FWC with the help the circulator 72 and the twoinput and output FWCs.

All of the components/devices belonging to the 2D-ROCDM may also bedirectly fabricated and embedded on the photonic die of the PIC. Anexternally incoming optical channel from a remote link partner TXj(λl)may enter the multi-λDWDM TRX 100 through either the EAST ADM port orthe WEST ADM port. The incoming optical channels are guided by theOWG-TE that plays the role not only to carry the channels but also tofilter unwanted the TM modes and the high order of TE modes from theincoming beams. The MRR(λl)s belonging to 2D-ROCDM can drop any desiredoptical channel from an incoming optical signal and direct them into aselected receiver RXj(λl), by applying suitable heat on drop micro-ringresonator heaters. The channels passing through MRR(λl)s are directed tothe links either on the West Path or on the EAST path that has thebuilt-in RX-VOAs, which are used to balance the optical power depositedon all RX-PDs.

FIG. 4 illustrates an optical transceiver 150 according to a furtherembodiment of the invention. The optical transceiver 150 of thisembodiment is similar to the optical transceiver 10 described above withreference to FIG. 1, but the optical transceiver 150 of this embodimentcomprises only a single optical transmitter 18, a single reconfigurableoptical channel-add apparatus 20, a single optical receiver 32, and asingle reconfigurable optical channel-drop apparatus 34. It will beunderstood therefore that the optical transceiver 150 is a single-λoptical transceiver.

A further embodiment of the invention provides an optical transceiver160 as shown in FIG. 5, which again is a single-λ optical transceiverand is substantially the same as the previous embodiment. The opticaltransceiver 160 of this embodiment is implemented as an integrated,single photonic integrated circuit, PIC.

The optical transceiver 160 of this embodiment is similar to the opticaltransceiver 70 described above with reference to FIG. 3, but the opticaltransceiver 160 of this embodiment comprises only a single opticaltransmitter 82, a single reconfigurable optical channel-add apparatus84, a single optical receiver 110, and a single reconfigurable opticalchannel-drop apparatus 112.

As with the multi-λ transceiver 70, the PIC single-λ opticaltransceiver, TRX, 160 may be packaged in one of various well-known smallform factor pluggable modules, such as SFP/SFP+ or may be designed as aboard-mounted module using proprietary types of form factors for PICpackaging.

An embodiment of the invention provides a radio base station, RBS, node200 as illustrated in FIG. 6. The RBS node comprises a multi-wavelengthoptical transceiver 10, as described above. It will be appreciated thatthe RBS 200 may comprise any of the multi-wavelength opticaltransceivers 10, 50, 70 of FIGS. 1 to 3.

An embodiment of the invention provides a radio base station, RBS, 250as illustrated in FIG. 7. The RBS comprises a remote radio unit, RRU,252 a baseband unit, BBU, 254, a first optical fibre link 256 and asecond optical fibre link 258.

The RRU 252 comprises a first single-λ optical transceiver 150, asdescribed above with reference to FIG. 5. The BBU 254 comprises a secondsingle-λ optical transceiver 150, as described above with reference toFIG. 5. It will be understood that a single-λ optical transceiver 160,as described above with reference to FIG. 6, may alternatively be used.

The first single-λ optical transceiver 150 is operable to generate anoptical channel at a first wavelength, λk, and the second single-λoptical transceiver 150 is operable to generate an optical channel at asecond wavelength, λl, different to the first wavelength.

The first and second optical fibre links 256, 258 are coupled betweenthe RRU and the BBU. The first optical fibre link 256 forms a workingmode, WM, link and the second optical fibre link 258 forms a protectionmode, PM, link.

The single-λ transceiver 150 has built-in switching functionality andcan therefore support two bi-directional wavelengths over one of the tworedundant fibre links, either the WM link or the PM link. The single-λtransceiver 150 therefore supports deployment of link protectionmechanisms in point-to-point connected two host systems, such as the RBS250, used in legacy 2G/3G/4G networks.

An embodiment of the invention provides an optical communicationsnetwork 300, as illustrated in FIG. 8. The network 300 comprises abi-directional wavelength division multiplexing, WDM, ring 302interconnecting a plurality of first RBS nodes 310, 320 according to theprevious embodiment. At least one of the first RBS nodes is a basebandunit, BBU, 310 and at least one other of the first RBS nodes is a remoteradio unit, RRU 320.

The WDM ring consists of optical fibre 302 in a ring configuration thatfully interconnects the RBS nodes 310, 320 via the multi-wavelengthtransceivers, TRX, 10. The bi-directional WDM ring has a working mode,WM, in which downstream, DS, channels are transmitted in acounter-clockwise direction and upstream, US, channels are transmittedin a clockwise direction, and a protection mode, PM, in whichdownstream, DS, channels are transmitted in the clockwise direction andupstream, US, channels are transmitted in the counter-clockwisedirection.

An embodiment of the invention provides an optical communicationsnetwork 330, as illustrated in FIG. 9. The network 330 is similar to thenetwork 300 of the previous embodiment, with the addition of a pluralityof second RBS nodes 350, 360 and a plurality of reconfigurable opticaladd drop multiplexers, ROADMs, 340.

The second RBS nodes 350 are legacy RBS nodes, each comprising asingle-wavelength optical transceiver, single-λDWDM TRX, 352. At leastone of the second RBS nodes is a baseband unit, BBU, 350 and at leastone other of the second RBS nodes is a remote radio unit, RRU 360.

Each legacy BBU 350 and each legacy RRU 360 is connected to the WDM ring302 via a respective ROADM 340. That is to say, unlike the BBU 310 andthe RRU 320 having the multi-wavelength TRX 10, there is no embeddedROADM in the legacy RBS nodes.

In an embodiment, the optical communications network 330 is an RBS siteand the WDM-ring 302 is a DWDM-ring. For simplicity, FIG. 9 shows only asingle RBS site being deployed over the bi-directional DWDM-Ring withsimplex design for the transport of both CPRI and OAM data streams. Itwill be understood that network 330 can be scaled up from a single RBSsite to a plurality of RBS sites, to form an entire fronthaul opticalnetwork of a radio access network, RAN.

The first RBS nodes 310, 320 are 5G RBS nodes and the second RBS nodes350, 360 are either 2G, 3G or 4G nodes.

The fibre infrastructure used by DWDM-Ring follows the classic simplexdesign using a single SMF to carry bi-directional traffic for bothupstream, US, and downstream, DS, traffic. The use of such aninfrastructure implies that, to avoid data collision, the wavelengthsselected to carry both the upstream channels and the downstream channelsshall be different. For example, one may use the odd numbers ofwavelengths for the upstream channels and the even numbers ofwavelengths for the downstream channels respectively to schedule thechannel plan with the wavelengths belonging to the desired DWDM bands,such as the C-band or L-band.

The RBS site 330 comprises 4 interconnected sub-sites 310, 320, 350,360. The nodes belonging to two of the four sub-sites are equipped witha multi-λ TRX 70, as described above. These are the 5G-Radios and themulti-λ TRX enabled BBUs 310. Two external ROADMs 340 are used tosupport access to the DWDM-Ring for the other two sub-sites, one ofwhich comprises legacy BBU-clusters 350 that communicate with the5G-Radios and the other sub-site comprises legacy Radios 360 that areremote link partners of multi-λ TRX enabled BBUs. The external ROADMsmay be a mini-ROADM as described in WO2015/176764. Following convention,the transport directions for the upstream and the downstream data aredefined from the sub-sites of Radios to the sub-sites of BBUs. For theconvenience of discussion, the traffic flows along the clockwise andanticlockwise directions in the bi-directional DWDM-Ring are specifiedas the working mode for the upstream and the downstream datarespectively. It is understood that, in the protection mode, for exampleif an SMF in the DWDM-Ring is broken, the traffic flows for the upstreamand the downstream data will simultaneously be redirected to theopposite directions inside the DWDM-Ring.

For the 5G-Radio sub-site, each individual 5G-Radio 320 is equipped withthe multi-λ TRX 70. Via the embedded ROADM on the multi-λ TRX, the5G-Radios can be interconnected with each other in a cascaded-chain overthe DWDM-Ring. Following the convention in radio design, an “in-band”remote OAM system may be implemented for each individual 5G-Radio.“Remote” here refers to the fact that the OAM system implemented in the5G-Radio 320 is a “slave” system that can be remotely controlled by a“master” OAM system implemented in the related BBU-cluster and/or a corenetwork, and the “in-band” refers to the transport protocols used byremote OAM implemented on a data link layer and/or a network transportlayer, which is carried by CPRI data streams. With such an approach,there is no need to assign dedicated wavelengths to carry the OAMsignals over the DWDM-Ring.

The sub-site with legacy BBU-clusters 350 comprises an external ROADM340 that is used to add or drop desired channels from the BBU-clusters.An “out-band” remote OAM system may be implemented on the ROADM. Again,this OAM system is “slave” and its configurations/operations areremotely carried by the OAM system of BBU-clusters and/or the corenetwork. The “out-band” refers to the fact that the transport protocolsused by the remote OAM are not carried by the channels to transport theCPRI data streams. To be able add and drop OAM signals to/from both hostsystems of ROADM and BBU-cluster, a specific fibre link may be allocatedto transport the OAM signals between the host system of ROADM, ROADMnode, and the host system of BBU-cluster, RBS node. Besides thefunctions for channel add and drop, the ROADM can also perform channelsbypass and/or redirect traffic on either direction over the DWDM-Ring.

A number of BBUs 350 that support the LTE standard, i.e. 4G, may be usedto construct the legacy BBU-clusters. The BBUs belonging to such acluster are configured for communication with each other as a singlemulti-task master, which can share their baseband resources for dataprocessing, and bridge the 5G-Radios to the core network so that varioustypes of services can be delivered to UEs by the CN. Both “in-band” and“out-band” OAM systems are implemented for the BBU-cluster 350 toremotely manage the OAM operations on both the 5G-Radios 320 and theexternal ROADM 340.

The sub-site with legacy Radios 360 also comprises an external ROADM 340similar to that used by the sub-site with BBU-clusters. Here, the ROADMis used to add or drop desired channels from the legacy Radios, andbridges data transport between legacy Radios 360 and multi-λ TRX enabledBBUs 310. This ROADM is also equipped with “out-band” OAM. Because theOAM signals have to be carried over the DWDM-Ring, a dedicated OAMchannel, for example an express channel on the ROADM, with two specificwavelengths, may be allocated to bi-directionally transport OAM signalsbetween ROADM node 340 and the host system of the multi-λTRX enabledBBU-cluster 310. The legacy Radios sub-site may comprise different typesof legacy radios used by the 2G/3G/4G networks. For each individuallegacy radio, a DWDM-enabled single-λ TRX 352, for example aphotonically enabled DWDM SFP+ module, may be implemented to communicateon a respective channel belonging to the multi-λ TRX of a respectivemulti-λ TRX enabled BBU 310.

For the sub-site with the multi-λ TRX enabled BBUs 310, legacypoint-to-multipoint break-out CPRI transport configuration between asingle BBU and a number of legacy RRUs can be recovered, and the BBU 310can be directly connected to the DWDM-Ring 302 without involving anylegacy DWDM access devices, such as transponders and arrayed waveguidegratings, AWGs. Compared to legacy transceivers, such as standard SFP+modules, there are many advantages in the use of the multi-λDWDM TRX,such as a reduction of manufacturing cost and energy consumption,reduction of front-panel space occupied by a number of SFP+, andsimplified mechanical design to fulfil the requirements ofelectromagnetic compatibility.

Following the legacy design of BBUs, an “in-band” OAM handling system isalso implemented for each individual multi-λ TRX enabled BBU 310 toremotely manage OAM operations on related legacy 2G/3G/4G Radios. Forremotely handling “out-band” OAM operations on the ROADM, a dedicatedOAM remote system may also be implemented, which may use the expresschannels dedicated for OAM handing for the ROADM node 340.

Since all nodes being used to construct the cascaded link chain over theDWDM-Ring are the type of ordinary ROADMs or the nodes with the embeddedROADM, the process for link protection can be implemented. It meansthat, if the SMF between two adjacent nodes or the SMF for the transportlinks between two sub-sites is broken, the loss of bi-directionaltraffic for both the upstream and the downstream data running with theworking modes inside the DWDM-ring can be recovered by redirecting thebi-directional traffic to run the protection mode over the DWDM-Ring.Such a link redundancy mechanism ensures the safe and robust transportof both upstream and downstream traffic among the nodes and/or thesub-sites over the DWDM-Ring.

Each multi-λ TRX 70 is equipped with multi-λDWDM transmitters TXi(λk) 80and multi-λ receivers RXj(λl) 110. Each single-λ TRX 352 is equippedwith single-λDWDM transmitter, TXj(λl), and a single-λ receiver RXj(λl).The indexes; i=1, 2, . . . , M and j=1, 2, . . . , N; are opticalchannel marks, which may be statically assigned to all transmitters andall receivers belonging to both the multi-λ TRXs 70 and the single-λTRXs 352, and the indexes; k=1, 2, . . . , m, l=1, 2, . . . , n and k≠l;are the wavelength marks. The summation of “m+n” is the total number ofwavelengths planned for the DWDM-Ring 302. The total number of channels“M+N” may be equal to the total number of wavelengths “m+n” for thechannel planning.

For the transmitter channels, TXi(λk) and TXj(λl), the wavelengths λkand λl may be statically assigned to both the multi-λ TRXs 70 and thesingle-λDWDM TRXs 352 according to a pre-defined channel-plan. For thereceiver channels, RXi(λk) and RXj(λl), any of wavelengths λk and λl maybe dynamically assigned to any of desired channels during channel-dropoperations carried by the embedded ROADMs of the multi-λ TRXs 70 and/orthe standalone ROADMs 340 connecting the single-λDWDM TRXs 352.

The TXi(λk) are configured for adding DWDM channels belonging to themulti-λ TRX into the DWDM-Ring and further transported to the DWDMreceivers RXi(λk) belonging to the single-λDWDM TRXs.

The optical receivers, RXj(λl), 110 of the multi-λDWDM TRX 100 supportdropping of DWDM channels from the DWDM-Ring 302, which are transmittedby the optical transmitters, TXj(λl), of the single-λDWDM TRXs 352. Theoptical receivers, RXj(λl), 110 of the multi-λDWDM TRX 100 also supportbypassing DWDM channels that are not to be dropped to the multi-λDWDMTRX 100 and returning them back into the DWDM-Ring. Generally speaking,the optical receivers, RXj(λl), 110 of the multi-λDWDM TRX 100 may havethe same amount of channel numbers as the optical transmitters, TXi(λk),80.

Each optical receiver, RXj(λl), comprises a receiver photodiode, RX-PD,and is coupled to a reconfigurable optical channel-drop apparatus, whichin this embodiment is a 2-directional reconfigurable opticalchannel-drop multiplexer ROCDM, 2D-ROCDM. The 2D-ROCDM comprises: anOPC; and two forming the first (‘West’) drop path 114 and the second(‘East’) drop path 118, including two variable optical attenuators,RX-VOA, 116, 120 for receiver power regulation; a drop micro-ringresonator 130, MRR(λl). The RX-PDs may be wideband devices that cancover an entire wavelength band, for example the C-band, so that theRX-PDs can support the selective drop of any desired wavelengthstransmitted by the transmitters, TXj(λl), in the single-λDWDM TRXs 352to any desired receiver channel, RXj(λl), multi-λDWDM TRX 100 via the2D-ROCDM 112.

Each ADM port 72, 74 is coupled to the respective FWCs via two SMFs.Each ADM port comprises a 3-way optical circulator which splits incomingtraffic and/or combines outgoing traffic. Due the nature of circulatordesign, it can also reject any backward propagating signals to avoidrecycling of dropped channels back out of the optical transceiver 100into the DWDM-ring 302.

The centre wavelengths of optical transmitters may be selected accordingto a proprietary channel-plan and/or the standardized DWDM channel-planspecified by standardization bodies, for example ITU-T G.694.1 06/2002,which belongs to one of the SMF transmission windows, e.g. C-band orL-band, etc. It is understood that the channel-plan will be determinedby the total wavelengths “m+n” specified to both the TXi(λk) of themulti-λDWDM TRXs 100 and the TXj(λj) of the single-λDWDM TRXs 352, witha well-specified channel-spacing, e.g. 100 GHz, 50 GHz, 25 GHz etc.

During the normal operation of the multi-λDWDM TRX 100, outgoing opticalchannels may only be guided either on the WEST ADM Port or the EAST ADMPort. This means that both upstream and downstream traffic shall alwaysbe transported in the opposite direction over the DWDM-Ring 302. Forexample, if the system decides to add channels onto the WEST ADM port,the maximum attenuation will be set by the right TX-VOA 92 on the “East”add path 90 to completely terminate the beam onto the EAST ADM port 74.Similarly, if the system decides to add channels onto the EAST ADM Port,the maximum attenuation will be set by the left TX-VOA 88 on the “WEST”add path 86, to completely terminate the beam onto the WEST ADM port. Inorder to effectively terminate unwanted channels into the DWDM-Ring, theTX-VOA is used as the outgoing beam terminators for the multi-λDWDM TRX100.

Another embodiment of the invention provides an optical communicationsnetwork 370, as illustrated in FIG. 10. The network 370 is similar tothe network 330 of the previous embodiment, with the addition of anetwork controller 380 and an OAM system in the core network 390.

The network controller comprises a base station controller, BSC, and aradio network controller, RNC, configured to obtain a reference opticalpower for the communications network 370.

The optical attenuator control apparatus, TX-VOA Ctrl, 52 in the BBUnodes 310 are configured to receive a third control signal comprising anindication of the reference optical power.

The network controller 380 is configured to: obtain a transmission lossfor each optical channel of each of the RBS nodes 310, 320, 350, 360;identify the optical channel having the maximum transmission loss; andset the reference optical power equal to an optical power of the opticalchannel having the maximum transmission loss.

In an embodiment, both the sub-site with the legacy BBU-clusters 350 andthe sub-site with multi-λ TRX enabled BBUs 310 are further connected toa backhaul transport network and the core network 390 through differentlink paths, depending on the types of mobile networks in use. For 2G and3G mobile networks, two cascaded link paths, A1 and A2, may be used tointerconnect the multi-λ TRX enabled BBUs 310, BSC/RNC 380 and the corenetwork. For 4G and 5G mobile networks, two sub-sites may be directlyconnected to the backhaul transport network and the core network vialink paths B or C, respectively. It is understood that UEs can beattached and/or have handover between legacy BBU-clusters and/or themulti-λ TRX enabled BBUs 310 through air interfaces of antennas (notshown) that are either directly integrated in or are external devicesconnected to both the 5G-Radios 320 and legacy Radios 360.

In an embodiment, the network controller 380 is configured to implementthe method 400, 410 of controlling optical powers of optical channels inan optical communications network according to any of the embodimentsdescribed below.

Referring to FIG. 11, an embodiment of the invention provides a method400 of controlling optical powers of optical channels in an opticalcommunications network comprising a plurality of nodes, such as the RBS250 and the optical communications networks 300, 330, 370 describedabove with reference to FIGS. 7 to 10.

The method comprises obtaining 402 a reference optical power;determining 404 an optical power of an optical channel generated by anoptical transmitter of a node; and applying 406 an attenuation to theoptical channel to reduce the optical power of the optical channel tothe reference optical power.

In a method 410 according to another embodiment, illustrated in FIG. 12,the reference optical power is obtained by: obtaining 412 a transmissionloss for each optical channel of each of the nodes; identifying 414 theoptical channel having the maximum transmission loss; and setting 416the reference optical power equal to an optical power of the opticalchannel having the maximum transmission loss.

A further embodiment of the invention provides a method of controllingoptical powers of optical channels in an optical communications network500 comprising a plurality of nodes 310, 320, 340, as illustrated inFIG. 13. The method of this embodiment is similar to the methods 400,410 of the previous embodiments.

During the propagation of optical signals inside the DWDM-Ring 302, thetransmission losses, TL, for each individual channel belonging to aspecific node 310, 320, 340 may increase with an increasing number ofbypassed nodes and the length of fibres used for node interconnection.After reaching the respective receiver, the total transmission loss foreach individual channel may also be significantly different if theoptical channel is propagated through the DWDM-ring cascaded node-chainalong the EAST link path (i.e. the clockwise link path) or along theWEST link path (i.e. the anticlockwise link path), as illustrated inFIG. 13.

Considering, for example, communication between ROADM node A0 340,connecting to a sub-site of legacy BBUs and 5G Radio nodes A1, A2, . . ., Ak, 320. In this example, let's assume that the corresponding TL forthe bypass of these nodes are TL_(A0), TL_(A1), TL_(A2), . . . , TL_(Ak)respectively, and that the TL due to the length of the fibres betweenthe four sub-sites are TL_(EW), TL_(WE), TL_(AB) and TL_(BA)respectively; we assume the loss inside a sub-site is negligible. Then,we observe that the nodes/channels belonging to A1 and Ak may have thelowest and highest TL if the direction of propagation is along the EASTlink path, i.e. the propagation direction for upstream traffic in theworking mode, and the total transmission loss for the channels belongingto node A1, TLtotal_((A1)), may be estimated byLtotal_((A1)) =TL _(EW)

while the total loss for the channels belonging to node Ak TLtotal(Ak)may be estimated byTLtotal(Ak)=TL _(EW) +TL _(A1) +TL _(A2) +, . . . +TL _(Ak-1).

If the propagated direction is changed to be along the WEST link path,the total loss for the nodes A1 and Ak will be significantly different,and may be estimated by:TL _(total(A1)) =TL _(BA) +TL _(WE) +TL _(AB) +TL _(A2) +TL _(A3) +, . .. ,+TL _(Ak) +TL _(B0) +TL _(B1) +TL _(B2) . . . ,+TL _(Bh)TL _(total(Ak)) =TL _(BA) +TL _(WE) +TL _(AB) +TL _(B0) +TL _(B1) +TL_(B2) . . . ,+TL _(Bh)

Since the TL for the bypassed nodes and the fibre length for nodeinterconnection will be approximately constant, the values of TL may becalibrated during manufacturing and saved as factory inventory data inmemory devices belonging to the nodes. In order to achieve initialequalization of optical power for all channels during their propagationover the DWDM-Ring, a simple “static power-balance method” may be used,which includes the determination of a reference optical power for allchannels and the adjustment of the optical power using TX-VOA for eachindividual channels according to the reference. Since the TX-VOAs canonly turn down the outgoing optical power, the channel with maximum TLmay be selected as a reference channel to set the reference opticalpower.

The process for TX power regulation may only be applied to the multi-λTRX, which implies that power regulation is not necessary for downstreamdata over the DWDM-Ring in both the normal working mode and theprotecting mode. This is because the TL due to bypassed nodes, thelength of fibre and the direction for beam propagation will be the samefor all single-λ TRX, in, for example, the nodes 350 belonging to thesub-site with legacy BBU-clusters and the sub-site 360 with legacyRadios.

It is understood that the method optical power regulation may only beapplied for compensating static loss sources, e.g. adding and removingnodes in the link paths of the DWDM-Ring. The method may not be appliedto compensate TL for losses caused by dynamic loss sources, such asmicro-bending induced losses on SMF due to significant changes ofenvironment, e.g. high temperature difference in summer and winterseasons. As the length of operating time increases, the optical powerfor each individual transceiver may be dynamically changed mainly due toperformance degradation of components belonging to the transceiver, e.g.the lasers. Real-time monitoring and dynamic changes to optical powerfor all channels and also real-time power regulation to rebalance theoptical power for all channels may therefore be required.

The method of controlling optical powers of optical channels accordingto this embodiment, which may be considered as a “power down-tuningmethod”, may be used for “real-time” power regulation of all channelsbeing propagated over the DWDM-Ring, which relies on the real-timemeasurement of optical power received by remote link partners, i.e. thereceivers RXi(λk) of the single-λDWDM TRX. The information of theoptical power detected by the receivers RXi(λk) can be extracted fromstandardized “diagnostic monitoring interface (DMI)” of the single-λTRX, as specified by SFP MSA, SFF-8472, The Diagnostic MonitoringInterface for Optical Transceivers, Rev 12.2 Nov. 21, 2014. DMI refersto the real-time diagnostic information stored in the memory of single-λTRX, which may include: real-time sensors, such as measurements of laserbias current, TX outgoing power, RX incoming power, module temperatureand supply voltages; control flags, such as TX disable, Rate select;status flags, such as TX fault, RX loss; warning flags and alarm flagsto indicate threshold being reached by the real-time sensors; andcustomer writable fields for dedicated/special applications.

With the method of this embodiment, the DMI information can be extractedby a host system of a single-λ transceiver via its two-wire serialinterface or management data input & output bus. The DMI information canbe remotely extracted and/or fetched using a remote OAM handling system.The DMI information can be used to instruct a host system of the multi-λTRX to perform fine tuning of optical power with the TX-VOAs. In such away, the dynamic power regulation for the multi-λ TRX can be achieved.

FIG. 14 illustrates a method according to a further embodiment ofcontrolling optical powers of optical channels in the opticalcommunications network 500.

In this embodiment, both “static” and “dynamic” power regulation can beimplemented for upstream data over the DWDM-Ring 302. The process starts420 with the boot and/or reboot of host systems, i.e. the ROADMs andnodes equipped with multi-λ TRX 70. In the next step, the processinvokes 422 inventory data stored in the memory devices 100 of themulti-λ TRX and/or the ROADMs 340. With the inventory data, anattenuation may be calculated 424 for each transmitter channel TXi(λk)80.

If the network 500 is configured to run the working mode 426, i.e. theupstream data is propagated along the “EAST link path”, the methodcomprises setting 430 a maximum value of attenuation on the TX-VOA 88 inthe West Path 86 of the 2D-ROCAM 84. A reference optical power P{Ref} isthen determined, by comparison of total TL for all nodes/sites deployedinside the DWDM-Ring, and determining the node having the maximum TL asthe reference. With the reference power, an estimated attenuation is setfor the rest of nodes to equalize the optical power for all channels,using the TX-VOA 92 in the “East Path” 90 of the 2D-ROCAM 84.

If the network is switched from the working mode to the protection mode,i.e. the beam propagation is through the “WEST link path”, the methodcomprises setting 428 a maximum value of attenuation on the TX-VOA 92 inthe East Path 90 of the 2D-ROCAM 84. The reference optical power P{Ref}is then redetermined by the comparison of total TL for all nodes/sitesdeployed inside the DWDM-Ring, determining the node having the maximumTL as the reference, and setting the estimated attenuations for the restof nodes to equalize the optical power for all channels using the TX-VOA88 in the “West Path” 86 of the 2D-ROCAM.

To dynamically regulate the optical power of the channels, the methodcomprises fetching 432 stored remote DMI information from the memorydevices of the single-λDWDM TRX 352; the real-time measured referenceoptical power P{Ref} of the reference channel, i.e. the channel with thelowest optical power among all receiver channels RXi(λk), can beobtained. By comparing the attenuation difference between the referencechannel and the rest of the optical channels, the method compensates anyattenuation difference for each transmitter channel TXi(λk) of themulti-λ TRXs 70 using the respective TX-VOA.

The method further comprises checking 434 whether there is a significantchange of optical power. This is done by subtracting the measuredoptical power of each individual channel P{RXi(λk)} from the referenceoptical power P{Ref}, and comparing the difference to a predefinedthreshold Pthr. If the power difference is less than Pthr, the methodmay go to a “standby” status with a “sleep time” T(sleep) 436, which maybe varied as necessary. The power comparison will be executed again ifT=T(sleep) is exceed.

If |P{RXi(λk)}−P{Ref}|<=Pthr is not satisfied, the method compriseschecking whether this is due to a change of operating mode 438, changingthe working mode to/from the protection mode. If it is not, the methodwill only perform “dynamic” power regulation to balance the opticalpower for all channels. Otherwise, the method will start over again andperform both “static” and “dynamic” power regulation. Before invoking“static” and “dynamic” power regulation, the method will first check 440if the TX-VOA 88, 92 of the multi-λ TRXs 70 have reached the maximumtuning range. If the maximum tuning range is not reached, the “static”and “dynamic” power regulation will be executed. If one of the TX-VOAsis beyond its tuning limit, the method will generate 442 an alarm signalto indicate a failure of power regulation to the OAM handling system,and will terminate.

Referring to FIG. 15, a further embodiment provides method ofcontrolling optical powers of optical channels in the opticalcommunications network 500 in which optical power control is alsoperformed at the receivers of the multi-λ TRX enabled BBUs 310.

In this embodiment, “power down-tuning” is also used for “real-time”power regulation on the local RX-PDs of the multi-λ TRX 70, which relieson real-time measurement of the optical power for all receivers RXj(λl)110. The method comprises extracting and comparing measured values ofoptical power at the RX-PDs, to identify and specify the referencechannel, i.e. the channel that has the lowest optical power. The RX-VOAs116, 120 are then configured to adjust the optical power on the rest ofchannels until they reach the same level of optical power as thereference channel. In this way, the optical power deposited on theRX-PDs may be dynamically balanced.

This method of “RX power down-tuning” may be implemented using theRX-VOA and RX-VOA Ctrl. After booting or rebooting of systems and/ornodes equipped with multi-λ TRX 70, the method comprises specifyingdefault values, including setting minimum attenuation values for RX-VOA,and defining a threshold value optical power, Pthr. The method theperforms real-time measurements of optical power for each individualchannel, determines the reference channel having the lowest opticalpower at the respective RX-PD, and takes the value of the lowest opticalpower as the reference optical power, P(Ref). The P(Ref) is then appliedto the rest of the channels by configuring the RX-VOAs to haveappropriate attenuations. This is done by subtracting the measuredoptical power for each individual channel P{RXj(λk)} from the referenceoptical power P{Ref}, and compared the difference to the predefinedthreshold Pthr. If the difference is less than Pthr, the method enters a“standby” status with a “sleep time” T(sleep), which may be varied asnecessary. The method repeats the power comparison again if T=Ti(sleep)is exceed.

If the condition given in equation IP{RXj(λk)}−P(Ref)|<=Pthr is notsatisfied, the method comprises checking if the RX-VOAs are beyond theirtuning limit.

If the limit of the tuning range of the RX-VOA is not reached, theprocess will start over again by measuring the optical power for allchannels, determining a new reference channel and setting the opticalpower for all channels to be the same as that of the reference channel.If an RX-VOA is out of its tuning range, the method comprises generatingan alarm signal to inform the OAM system of the failure and the methodwill terminate.

The invention claimed is:
 1. A method of controlling optical powers ofoptical channels in an optical communications network comprising aplurality of nodes, the method comprising: determining whether ameasured optical power has satisfied a pre-determined threshold;responsive to determining the measured optical power has satisfied thepre-determined threshold, determining whether the measured optical poweris due to a change of an operating mode of the optical communicationsnetwork; responsive to determining the measured optical power is not dueto a change of an operating mode of the optical communication network,determining whether a maximum tuning range of an optical attenuator hasbeen reached; responsive to determining the maximum tuning range has notbeen reached, obtaining a reference optical power; determining anoptical power of an optical channel generated by an optical transmitterof a node; and applying an attenuation to the optical channel to reducethe optical power of the optical channel to the reference optical power.2. The method as claimed in claim 1, wherein the optical communicationsnetwork is a fronthaul network of a radio access network, RAN, and thenodes are radio base station (RBS) nodes.
 3. The method as claimed inclaim 1, wherein the method is performed by a network controlleroperating in the optical communications network.
 4. The method asclaimed in claim 1, wherein determining the optical power of the opticalchannel comprises determining an optical power of an optical channelgenerated by a multi-wavelength optical transmitter of an RBS node. 5.The method as claimed in claim 1, wherein the plurality of nodescommunicate via a dense wavelength division multiplexing (DWDM) ring. 6.The method as claimed in claim 1, wherein the optical communicationsnetwork comprises a 5G radio network and one of a 2G, 3G, and 4G radionetwork.
 7. A network controller configured to control optical powers ofoptical channels in an optical communications network comprising aplurality of nodes, the network controller comprising: processingcircuitry configured to perform operations comprising determiningwhether a measured optical power has satisfied a pre-determinedthreshold; responsive to determining the measured optical power hassatisfied the pre-determined threshold, determining whether the measuredoptical power is due to a change of an operating mode of the opticalcommunications network; responsive to determining the measured opticalpower is not due to a change of an operating mode of the opticalcommunication network, determining whether a maximum tuning range of anoptical attenuator has been reached; responsive to determining themaximum tuning range has not been reached, obtaining a reference opticalpower, determining an optical power of an optical channel generated byan optical transmitter of a node, and applying an attenuation to theoptical channel to reduce the optical power of the optical channel tothe reference optical power.
 8. The network controller as claimed inclaim 7, wherein the optical communications network is a fronthaulnetwork of a radio access network, RAN, and the nodes are radio basestation (RBS) nodes.
 9. The network controller as claimed in claim 7,wherein the processing circuitry is further configured to determine theoptical power of the optical channel by determining an optical power ofan optical channel generated by a multi-wavelength optical transmitterof an RBS node.
 10. The network controller as claimed in claim 7,wherein the plurality of nodes are connected via a dense wavelengthdivision multiplexing (DWDM) ring.
 11. The network controller as claimedin claim 7, wherein the optical communications network comprises a 5Gradio network and one of a 2G, 3G, and 4G radio network.
 12. A computerprogram product comprised on a non-transitory computer readable medium,the computer program product comprising executable instructions thatwhen executed by a processor of a network controller of a networkcontroller configured to control optical powers of optical channels inan optical communications network comprising a plurality of nodes,causes the processor performs operations comprising: determining whethera measured optical power has satisfied a pre-determined threshold;responsive to determining the measured optical power has satisfied thepre-determined threshold, determining whether the measured optical poweris due to a change of an operating mode of the optical communicationsnetwork; responsive to determining the measured optical power is not dueto a change of an operating mode of the optical communication network,determining whether a maximum tuning range of an optical attenuator hasbeen reached; responsive to determining the maximum tuning range has notbeen reached, obtaining a reference optical power; determining anoptical power of an optical channel generated by an optical transmitterof a node; and applying an attenuation to the optical channel to reducethe optical power of the optical channel to the reference optical power.13. The computer program product as claimed in claim 12, wherein theoptical communications network is a fronthaul network of a radio accessnetwork, RAN, and the nodes are radio base station (RBS) nodes.